A. Field of the Invention
The present invention relates generally to scanning systems. More particularly, this invention relates to the use of imagers to create of xerographic images.
B. Description of the Related Art
Electrophotographic printers wherein a laser scan line is projected onto a photoconductive surface are well known. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of signals to be imaged on a precharged photoreceptor (a photosensitive plate, belt, or drum) for purposes of xerographic printing. The ROS provides a laser beam which switches on and off according to digital image data associated with the desired image to be printed as the beam moves or scans, across the photoreceptor. Commonly, the surface of the photoreceptor is selectively, imagewise discharged by the laser in locations to be printed white, to form the desired image on the photoreceptor. The modulation of the beam to create the desired latent image on the photoreceptor is facilitated by digital electronic data controlling a modulator associated with the laser source. A common technique for effecting this scanning of the beam across the photoreceptor is to employ a rotating polygon surface (the surface of the polygon typically being a mirror or other reflective surface); the laser beam from the ROS is reflected by the facets of the polygon, creating a scanning motion of the beam, which forms a scan line across the photoreceptor. A large number of scan lines on a photoreceptor together form a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with a toner, and the developed image is transferred to a copy sheet and fixed, as in the well-known process of xerography.
FIG. 1 shows the basic configuration of a scanning system 100 used, for example, in an electrophotographic printer or facsimile machine. A laser source 10 produces a collimated laser beam, also referred to as a "writing beam", 12 which is reflected from the facets of a rotating polygon 14. Each facet of the polygon 14 in turn deflects the writing beam 12 to create an illuminated beam spot 16 on the pre-charged surface of photoreceptor 18. The system may further include additional optical elements such as focusing lenses. The energy of the beam spot 16 on a particular location on the surface of photoreceptor 18, corresponding to a picture element (pixel) in the desired image, discharges the surface for pixels of the desired image which are to be printed white. In locations having pixels which are to be printed black, the writing beam 12 is at the moment of scanning interrupted, such as by a modulator 11 controlled by imagewise digital data, so the location on the surface of photoreceptor 18 will not be discharged. It is to be understood that gray levels are typically imaged in like manner by utilizing exposure levels intermediate between the "on" and "off" levels. Thus, digital data input into laser source 10 is rendered line by line as an electrostatic latent image on the photoreceptor 18.
When the beam spot 16 is caused, by the rotation of polygon 14, to move across photoreceptor 18, a scan line 20 of selectively discharged areas results on photoreceptor 18. In FIG. 1, the photoreceptor 18 is shown as a rotating drum, but those skilled in the art will recognize that this general principle, and indeed the entire invention described herein, is applicable to situations wherein the photoreceptor is a flat plate, a moving belt, or any other configuration. The surface of photoreceptor 18, whether it is a belt or drum, moves in a process direction (as indicated by the arrow drawn on the side of the drum 18); the motion of spot 16 through each scan line 20 is transverse to the process direction (as indicated by the arrow drawn on the surface of the drum 18 and below scan line 20). The periodic scanning of beam spot 16 across the moving photoreceptor 18 creates an array of scan lines 20, called a raster 22, on the photoreceptor 18, forming the desired image to be printed. One skilled in the art will appreciate that such a configuration will typically further include any number of lenses and mirrors to accommodate a specific design.
In a rotating-polygon scanning system, there is a practical limit to the rate at which digital information may be processed to create an electrostatic latent image on a photoreceptor. One practical constraint on the speed of a system is the maximum polygon rotation speed. It can be appreciated that high quality images require precision placement of the raster scan lines as well as exact timing to define the location of each picture element or pixel along each scan. In a conventional polygon scanner, this precision is achieved by holding very close mechanical tolerances on the polygon geometry and the rotational bearings supporting the polygon body and drive motor. Experience has shown that beyond about 20,000 RPM, precision ball bearings with the required closeness of fit have limited life and are impractical in many scanner applications. As a result, exotic alternatives such as air bearings are sometimes used, but these represent a substantial increase in engineering complexity and maintenance, and hence cost. Another constraint is the size of the polygon itself; it is clear that the forces associated with high speed rotation increase with the diameter of the object being rotated. In particular, both the stored energy and the gyroscopic forces that must be restrained by the bearings increase with the square of the diameter. It is therefore prudent to limit the polygon size to maximize bearing life as well as reduce the potential for damage should a bearing fail at high speed.
In addition to practical constraints, the speed of a printer must be considered in conjunction with other competing desirable characteristics of a printer, particularly resolution. In purely optical terms, there is a trade-off between speed and resolution in a scanning system. The higher the resolution, that is, the more pixels that are designed to form a latent image of a given size, the lower the numerical aperture of the optical system required in order to define the pixels accurately. This trade off can be summarized by a derived equation for an under filled system relating the angular velocity .omega. of a polygon having a mean diameter D to the desired pixel size (that is, the inverse of resolution) .DELTA.x: EQU D.omega..sup.2 =[L.lambda.P.sup.2 /.DELTA.x.sup.3 ][(60/.pi.).sup.2 k/2][E/.chi.]
The other variables in this equation are as follows: L is the length of the intended scan path, which in this context is the width of the photoreceptor across which the scan line is formed. P is the process speed, in inches per second, of the photoreceptor motion in the machine. k is a constant which depends on the intensity profile of the beam (for example, under a certain convention, the usable pixel size is dependent on a focused concentration where 86% of the total power of the beam is focused within a circular area of a given size). E is an efficiency, factor relating to the proportion of the "circumference" of the polygon which is practically usable for scanning purposes, i.e., because the numerical aperture for a given resolution .DELTA.x requires a specific beam width at the polygon, the beam will not be reflected usefully for a certain portion of the time when the beam is focused near the ends of the facets of the polygon. The larger the ratio of facet length to beam width, the larger the proportion of the polygon rotation which is usable for scanning purposes. .chi. is the ratio of reflected scan angle to rotational scan angle, which depends on whether the facets of the polygon are parallel or oriented at 45 degrees to the axis of the polygon. If the facets are parallel, as in the illustrated case, then .chi. is equal to
2. There are some designs in which the facets of the polygon are set at 45 degrees relative to the axis so that the polygon has the general appearance of a truncated cone. In that case, the beam from the source is incident on the facets parallel to the axis of the polygon, and is reflected in a direction perpendicular to the axis; for this geometry, .chi. is equal to 1.
Looking at the most important system design variables in the above equation, the scan length L, the process speed P, and the spot size .DELTA.x, it is clear that the desire for a longer scan, faster throughput, and higher quality image (smaller spot size) all increase the value of the right hand side of the equation and are at cross-purposes with the need to keep the left hand side of the equation, representing the demands on the polygon, as small as possible. As a practical matter, it has been discovered that for electrostatographic printers, the largest practical polygon from a cost and safety standpoint is one having a diameter of about five inches, although diameters of about two inches are generally preferred from a standpoint of machine compactness. Simultaneously, system cost and engineering difficulties are rapidly compounded at rotational speeds of more than 20,000 rpm. The above equation, it should be remembered, has been derived strictly on the basis of optical laws and without consideration of practical limitations. There is, therefore, a distinct advantage in any arrangement which facilitates a substantial increase in the possible rate of digital data that may be imaged with a scanning apparatus, thereby providing the possibility of enhanced resolution, increased scan length, or faster process speed, in various proportions without violating the necessary relationship defined in the equation.
Therefore, there is a need in the art for a system that can produce fast, wide images, without increasing the size of the imager nor its distance from the photoresistive material.